1. Field of the Invention
The present invention relates to the field of carbon filters and slurries, particularly activated carbon filters and slurries and the regeneration of used or spent activated carbon after use as a filter, precipitator or biologic reactor. Concentrated and active stable solutions of iodine are also described.
2. Background of the Art
Activated carbon, also called activated charcoal or activated coal is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions. The carbon may be provided by many different processes and in many of the various forms of carbon available, such as powdered carbon, expanded carbon, graphite, expanded graphite and the like.
The word activated in the name is sometimes replaced with active. Due to its high degree of microporosity, just 1 gram of activated carbon has a surface area in excess of 500 m2 (about one tenth the size of an American football field), as determined typically by nitrogen gas adsorption. Sufficient activation for useful applications may come solely from the high surface area, though further chemical treatment often enhances the adsorbing properties of the material. Activated carbon is usually derived from charcoal.
Activated carbon is carbon produced from carbonaceous source materials such as, by way of non-limiting examples, nutshells, peat, wood, coir, lignite, coal and petroleum pitch. It can be produced by one of the following non-limiting processes:                1. Physical reactivation: The precursor is developed into activated carbons using gases. This is generally done by using one or a combination of the following processes:                    Carbonization: Material with carbon content is pyrolyzed at temperatures in the range 600-900° C., in absence of oxygen (usually in inert atmosphere with gases like argon or nitrogen)            Activation/Oxidation: Raw material or carbonized material is exposed to oxidizing atmospheres (carbon monoxide, oxygen, or steam) at temperatures above 250° C., usually in the temperature range of 600-1200° C.                        2. Chemical activation: Prior to carbonization, the raw material is impregnated with certain chemicals. The chemical is typically an acid, strong base, or a salt (phosphoric acid, potassium hydroxide, sodium hydroxide, zinc chloride, respectively). Then, the raw material is carbonized at lower temperatures (450-900° C.). It is believed that the carbonization/activation step proceeds simultaneously with the chemical activation. Chemical activation is preferred over physical activation owing to the lower temperatures and shorter time needed for activating material.        
Activated carbons are complex products which are difficult to classify on the basis of their behavior, surface characteristics and preparation methods. However, some broad classification is made for general purpose based on their physical characteristics. They may be formally or informally characterized according to properties, method of production, morphology and/or other factors.
One form of activated carbon is known as powdered activated carbon (PAC). Activated charcoal under bright field illumination on a light microscope displays a fractal-like shape of the particles hinting at their enormous surface area. Each particle despite being only around 0.1 mm wide, has a surface area of several square meters.
Traditionally, active carbons are made in particulate form as powders or fine granules less than 1.0 mm in size with an average diameter between 0.15 and 0.25 mm. Thus they present a large surface to volume ratio with a small diffusion distance. PAC is made up of crushed or ground carbon particles, 95-100% of which will pass through a designated mesh sieve or sieve. Granular activated carbon is defined as the activated carbon being retained on a 50-mesh sieve (0.297 mm) and PAC material as finer material, while ASTM classifies particle sizes corresponding to an 80-mesh sieve (0.177 mm) and smaller as PAC. PAC is not commonly used in a dedicated vessel, owing to the high head loss that would occur. PAC is generally added directly to other process units, such as raw water intakes, rapid mix basins, clarifiers, and gravity filters.
Granular activated carbon is another form of activated carbon that has a relatively larger particle size compared to powdered activated carbon and consequently, presents a smaller external surface. Diffusion of the adsorbate is thus an important factor. These carbons are therefore preferred for all adsorption of gases and vapors as their rate of diffusion are faster. Granulated carbons are used for water treatment, deodorization and separation of components of flow system. GAC can be either in the granular form or extruded. GAC is designated by sizes such as 8×20, 20×40, or 8×30 for liquid phase applications and 4×6, 4×8 or 4×10 for vapor phase applications. A 20×40 carbon is made of particles that will pass through a U.S. Standard Mesh Size No. 20 sieve (0.84 mm) (generally specified as 85% passing) but be retained on a U.S. Standard Mesh Size No. 40 sieve (0.42 mm) (generally specified as 95% retained). AWWA (1992) B604 uses the 50-mesh sieve (0.297 mm) as the minimum GAC size. The most popular aqueous phase carbons are the 12×40 and 8×30 sizes because they have a good balance of size, surface area, and head loss characteristics.
Extruded activated carbon is another form that combines powdered activated carbon with a binder, which are fused together and extruded into a cylindrical shaped activated carbon block with diameters from 0.8 to 130 mm. These are mainly used for gas phase applications because of their low pressure drop, high mechanical strength and low dust content.
Impregnated carbon is a porous carbon containing several types of inorganic impregnant such as iodine, silver, cations such as Al, Mn, Zn, Fe, Li, Ca have also been prepared for specific application in air pollution control especially in museums and galleries. Due to antimicrobial/antiseptic properties, silver loaded activated carbon is used as an adsorbent for purification of domestic water. Drinking water can be obtained from natural water by treating the natural water with a mixture of activated carbon and Al(OH)3, a flocculating agent. Impregnated carbons are also used for the adsorption of H2S and thiols. Adsorption rates for H2S as high as 50% by weight have been reported.
Activated carbon is also available in special forms such as cloths and fibers. The “carbon cloth” for instance is used in personnel protection for the military.
A gram of activated carbon can have a surface area in excess of 500 m2, with 1500 m2 being readily achievable. Carbon aerogels, while more expensive, have even higher surface areas, and are used in special applications. FIG. 1 shows Activated carbon, as viewed by an electron microscope.
Under an electron microscope, the high surface-area structures of activated carbon are revealed. Individual particles are intensely convoluted and display various kinds of porosity; there may be many areas where flat surfaces of graphite-like material run parallel to each other, separated by only a few nanometers or so. These micropores provide superb conditions for adsorption to occur, since adsorbing material can interact with many surfaces simultaneously. Tests of adsorption behavior are usually done with nitrogen gas at 77 K under high vacuum), but in everyday terms activated carbon is perfectly capable of producing the equivalent, by adsorption from its environment, liquid water from steam at 100° C. and a pressure of 1/10,000 of an atmosphere.
Physically, activated carbon binds materials by van der Waals force or London dispersion force. Activated carbon does not bind well to certain chemicals, including alcohols, glycols, strong acids and bases, metals and most inorganics, such as lithium, sodium, iron, lead, arsenic, fluorine, and boric acid. Activated carbon does adsorb iodine very well and in fact the iodine number, mg/g, (ASTM D28 Standard Method test) is used as an indication of total surface area. Ammonia adsortion on activated carbon is both temperature and concentration dependent, directly, in aqueous liquids.
Carbon monoxide is not well absorbed by activated carbon. This should be of particular concern to those using the material in filters for respirators, fume hoods or other gas control systems as the gas is undetectable to the human senses, toxic to metabolism and neurotoxic.
Activated carbon can be used as a substrate for the application of various chemicals which improve the adsorptive capacity for some inorganic (and problematic organic) compounds such as hydrogen sulfide (H2S), ammonia (NH3), formaldehyde (HCOH), radioisotopes iodine-131(131I) and mercury (Hg). This property is known as chemisorption.
Iodine Number
Many carbons preferentially adsorb small molecules. Iodine number is the most fundamental parameter used to characterize activated carbon performance. It is a measure of activity level (higher number indicates higher degree of activation), often reported in mg/g (typical range 500-1200 mg/g). It is a measure of the micropore content of the activated carbon (0 to 20 Å, or up to 2 nm) by adsorption of iodine from solution. It is equivalent to surface area of carbon between 900 m2/g and 1100 m2/g. It is the standard measure for liquid phase applications.
Iodine number is defined as the milligrams of iodine adsorbed by one gram of a material such as carbon, organic materials (such as oils, lipids, hydrocarbons, carbohydrates, etc.) when the iodine concentration in the residual filtrate is 0.02 normal. Basically, iodine number is a measure of the iodine adsorbed in the pores and, as such, is an indication of the pore volume available in the activated carbon of interest. Typically, water treatment carbons have iodine numbers ranging from 600 to 1100. Frequently, this parameter is used to determine the degree of exhaustion of a carbon in use. However, this practice should be viewed with caution as chemical interactions with the adsorbate may affect the iodine uptake giving false results. Thus, the use of iodine number as a measure of the degree of exhaustion of a carbon bed can only be recommended if it has been shown to be free of chemical interactions with adsorbates and if an experimental correlation between iodine number and the degree of exhaustion has been determined for the particular application. Although carbon is primarily described herein, any other surface on a material (porous or not) may also be used as long as it can sustain or provide an iodine number of at least 100 mg/g. Silicone materials, polymers, composites, coated substrates (such as carbon coated, or graphite coated substrates) and the like are examples thereof. These materials are preferably porous or microporous to allow high surface areas per volume of material.
Dechlorination
Some carbons are evaluated based on the dechlorination half-value length, which measures the chlorine-removal efficiency of activated carbon. The dechlorination half-value length is the depth of carbon required to reduce the chlorine level of a flowing stream from 5 ppm to 3.5 ppm. A lower half-value length indicates superior performance.
Ash Content
Ash content reduces the overall activity of activated carbon. It reduces the efficiency of reactivation. The metal oxides (Fe2O3) can leach out of activated carbon resulting in discoloration. Acid/water soluble ash content is more significant than total ash content. Soluble ash content can be very important for aquarists, as ferric oxide can promote algal growths. A carbon with a low soluble ash content should be used for marine, freshwater fish and reef tanks to avoid heavy metal poisoning and excess plant/algal growth.
Carbon Tetrachloride Activity
Measurement of the porosity of an activated carbon by the adsorption of saturated carbon tetrachloride vapor.
Particle Size Distribution
The finer the particle size of an activated carbon, the better the access to the surface area and the faster the rate of adsorption kinetics. In vapor phase systems this needs to be considered against pressure drop, which will affect energy cost. Careful consideration of particle size distribution can provide significant operating benefits.
The most commonly encountered form of chemisorption in industry, occurs when a solid catalyst interacts with a gaseous feedstock, the reactant/s. The adsorption of reactant/s to the catalyst surface creates a chemical bond, altering the electron density around the reactant molecule and allowing it to undergo reactions that would not normally be available to it.
Carbon adsorption has numerous applications in removing pollutants from air or water streams both in the field and in industrial processes such as:                Spill cleanup        Groundwater remediation        Drinking water filtration        Air purification        Volatile organic compounds capture from painting, dry cleaning, gasoline dispensing operations, and other processes.        
Activated charcoal is also used for the measurement of radon concentration in air.
Activated carbon is also used as growth media in biologic methods of water and wastewater treatment.
Research is being done testing various activated carbons' ability to store natural gas and hydrogen gas. The porous material acts like a sponge for different types of gasses. The gas is attracted to the carbon material via Van der Waals forces. Some carbons have been able to achieve bonding energies of 5-10 kJ per mol. The gas may then be desorbed when subjected to higher temperatures and either combusted to do work or in the case of hydrogen gas extracted for use in a hydrogen fuel cell. Gas storage in activated carbons is an appealing gas storage method because the gas can be stored in a low pressure, low mass, low volume environment that would be much more feasible than bulky on board compression tanks in vehicles.
Filters with activated carbon are usually used in compressed air and gas purification to remove oil vapors, odors, and other hydrocarbons from the air. The most common designs use a 1 stage or 2 stage filtration principle in which activated carbon is embedded inside the filter media. Activated charcoal is also used in spacesuit Primary Life Support Systems. Activated charcoal filters are used to retain radioactive gases from a nuclear boiling water reactor turbine condenser. The air vacuumed from the condenser contains traces of radioactive gases. The large charcoal beds adsorb these gases and retain them while they rapidly decay to non-radioactive solid species. The solids are trapped in the charcoal particles, while the filtered air passes through.
Activated carbon is commonly used to purify homemade non-dangerous chemicals such as sodium acetate. Activated carbon, often impregnated with iodine or sulfur, is widely used to trap mercury emissions from coal-fired power stations, medical incinerators, and from natural gas at the wellhead. This carbon is a specialty product costing more than US$4.00 per kg. However, it is often not recycled, if it can be.
The regeneration of activated carbons involves restoring the adsorptive capacity of saturated activated carbon by desorbing adsorbed contaminants on the activated carbon surface. This has been practiced with a number of available techniques. The most common regeneration technique employed in industrial processes is thermal regeneration. The thermal regeneration process generally follows three steps:                Adsorbent drying at approximately 105° C.        High temperature desorption and decomposition (500-900° C.) under an inert atmosphere        Residual organic gasification by an oxidizing gas (steam or carbon dioxide) at elevated temperatures (800° C.)        
The heat treatment stage utilizes the exothermic nature of adsorption and results in desorption, partial cracking and polymerization of the adsorbed organics. The final step aims to remove charred organic residue formed in the porous structure in the previous stage and re-expose the porous carbon structure regenerating its original surface characteristics. After treatment the adsorption column can be reused. Per adsorption-thermal regeneration cycle between 5-15 wt % of the carbon bed is burnt off resulting in a loss of adsorptive capacity. Thermal regeneration is a high energy process due to the high required temperatures making it both an energetically and commercially expensive process. Plants that rely on thermal regeneration of activated carbon have to be of a certain size before it is economically viable to have regeneration facilities onsite. As a result it is common for smaller waste treatment sites to ship their activated carbon cores to a specialized facility for regeneration, increasing the process' already significant carbon footprint
http://www.prominentinc.com/cbac_impregnated_ki_nai.html discloses KI impregnated activated carbon.
KI/Potassium Iodide Impregnated Coal Based Activated Carbon.
It is effective for the desulphurization of gases and the removal of acidic contaminants such as hydrogen sulfide, hydrogen chloride, and mercaptans. The percentage of potassium iodine can be varied upon request.
AvailableUnitProducts RangeRemarksMesh SizeUSGAC: 4 × 6/or as requiredSieve4 × 8/4 × 10/8 × 20DiametermmPAC: 1.5/3.0/4.0or as requiredIodinemg/gMinimum1000or as requiredCCL4/CTC%Minimum60/65/70/75or as requiredKI Impregnation%Minimum2/3/5or as requiredH2S Breakg/ccMinimum0.14or as requiredThrough CapacityApparent Densityg/cc0.50-0.66or as requiredHardness%Minimum90/95or as requiredMoisture%Maximum15or as required
Chemical and Engineering News, Apr. 10, 2010, Volume 88, No. 6 “Wastewater Treatment,” Melody Voith, discloses a process for cleaning wastewater from paper plant manufacturing sites by adsorption of organic chemical wastes by passing low-voltage electric current through a graphite-based filter to cause electrochemical oxidation of organic on the particles. The carbon particles are first mixed with the wastewater top absorb the organic wastes. The current directly oxidizes the organic materials.